#include<vector>
using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(NULL) {}
};
 
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode(0);
        ListNode* res = head;

        while (l1 != NULL && l2 != NULL) {
            if (l1->val < l2->val) {
                head->next = l1;
                l1 = l1->next;
            }
            else {
                head->next = l2;
                l2 = l2->next;
            }
            head = head->next;
        }
        if (l1 != NULL)
            head->next = l1;
        if (l2 != NULL)
            head->next = l2;
        head = res -> next;
        delete res;
        return head;
    }
};